The present invention relates to optical assemblies for steering or redirecting light wherein light travels in forward and reverse directions through the steering assembly. One example of such a device is a scanning ophthalmic microscope, wherein light is directed along an input path at x- and y- axis steering mirrors before being focused at a target within the eye, and light reflected from the target is imaged back along substantially the same input path to the x- and y- axis steering mirrors before it is split off and directed to a photodetector or imaging device.
More generally, a clinical system may require that one or more input light beams and one or more observation beams all be steered together. In such a system, the input light may be a diagnostic illumination beam such as the infrared beam of a laser Doppler instrument, or a treatment beam such as a thin or point-focused surgical laser beam, directed along the steered path. Direct illumination for imaging light may also be provided independently of the mirror, for example, by flooding the eye with sufficient light to view the surgical field. The observation beam or beams may include return light which is focused to a visual image, transformed to a localized electrically-converted tracking image signal, converted to a tissue reflectance value for laser intensity control, or processed in some other way. For such an instrument, it is desirable that the treatment light and the collected return light both pass through steering mirrors to vary the field of treatment or observation, respectively.
One instrument of this type is described in U.S. Pat. No. 4,856,891 owned by the assignee of the present patent application. That patent discloses an ophthalmic instrument which steers a narrow diagnostic or treatment light beam, and receives a return image through a common steering system. The advantage of such a bidirectional steering system is that by moving the steering mirror or mirrors in a manner to stabilize the position of the return image, the input light path followed by the diagnostic or treatment beam is automatically maintained in a stable location on the eye fundus and that location bears a fixed spatial relation to the imaged area. However, when applying such a system to a target object on the fundus of the eye, scattering of the relatively intense input light in the steering assembly can add substantial noise to the extremely weak return signal. In addition, when it is desired to maintain a steering mirror and one or more stops in positions confocal with the observed field or with the pupil of the eye, precision is required in locating or aligning the beam with respect to these elements. This complicates the problem of maintaining different light systems sufficiently distinct, and further compounds the noise or crosstalk problems. When one of the mirrors is a scanning polygon and its faces undergo both translational and rotational motion, problems of varying optical path length, shifting mirror position and changing mirror incidence angle further complicate the task of forming a high quality image or maintaining several optical paths in registration with each other.
Accordingly, it is desired to provide a bidirectional optical steering system which corrects one or more of these shortcomings.